The present invention relates to floating type decanters for removing supernate from near the top of a liquid surface. In some installations, for example in the sequencing batch reactor process of waste water treatment, it is important to prevent entrance of material into the decanter during those times when the decanter is not being operated in decant mode. More particularly, in a sequencing batch reactor process of waste water treatment, a basin reactor is operated through successive fill, react, settle and decant phases and any suspended solids that enter the decanter during the fill, react and settle phases will be subsequently discharged with the decanted effluent during the decant phase and adversely affect the quality of the effluent. Different decanter designs have been made to overcome this problem. U.S. Pat. 4,601,833 discloses providing a floating decanter and lifting the decanter entirely our of the water in the basin during the fill, react and settle phases, with a trough to cover the inlet of the decanter during lifting of the decanter out of the basin and lowering of the decanter back into the basin to prevent entrance of floating debris. U.S. Pat. No. 4,695,376 discloses a floating decanter in which a decanter receptacle is disposed below a float and a power operated actuator is mounted on the float and connected to the decanter receptacle for selectively moving the decanter receptacle from a lower position in which the decanter rim is spaced below the underside of the float to a raised position to seal the decanter rim to the underside of float. Such decanters having power operated actuators present some problem as regards servicing particularly in closed tanks, and further present problems as regards use in tanks containing a combustible liquid or explosive atmosphere.